In the disclosure of the present invention reference is mostly made to the treatment of diabetes by injection of insulin, however, this is only a preferred use of the present invention.
Diabetes mellitus is the common name for at least 2 different diseases, one characterised by immune system mediated specific pancreatic beta cell destruction (insulin dependent diabetes mellitus (IDDM) or type 1 diabetes), and another characterised by decreased insulin sensitivity (insulin resistance) and/or a functional defect in beta cell function (non-insulin dependent diabetes mellitus (NIDDM) or type 2 diabetes).
The principal treatment of type 1 diabetes is straight forward substitution of the missing insulin secretion, whereas treatment of type 2 is more complicated. More specifically, in early stages of type 2 diabetes treatment a number of different types of drugs can be used, e.g. drugs which increase insulin sensitivity (ciglitazones), decrease hepatic glucose output (e.g. metformin), or reduce glucose uptake from the gut (alfa glucosidase inhibitors), as well as drugs which stimulate beta cell activity (e.g. sulfonylurea/meglitinides). However, the above-described deterioration is reflected in the fact that beta cell stimulators will eventually fail to stimulate the cell, and the patient has to be treated with insulin, either as mono therapy, or in combination with oral medication in order to improve glucose control.
Currently, there are two principal modes of daily insulin therapy, the first mode including syringes and insulin injection pens. These devices are simple to use and are relatively low in cost, but they require a needle stick at each injection, typically 3-4 times or more per day. The second mode is infusion pump therapy, which entails the purchase of a portable but relatively expensive pump, for which reason the initial cost of the pump is a barrier to this type of therapy. Although more complex than syringes and pens, the pump offer the advantages of continuous infusion of insulin, precision in dosing and optionally programmable delivery profiles and user actuated bolus infusions in connections with meals. Further, in combination with a blood glucose sensor an infusion pump may provide fully automatic closed loop control of insulin infusion.
The fluent medicine is usually contained in a reservoir, which can be either a user fillable reservoir or a pre-filled disposable reservoir. Such known reservoir can be provided as a cylindrical barrel having a penetrable membrane at a proximal end and a movable piston at the opposite distal end. A conduit penetrating the membrane is mounted at the proximal end.
When the piston is moved towards the membrane, the fluent medicament contained in the reservoir is pressed out through the conduit. When the conduit is mounted it is however possible for the medicament to escape out through the conduit and for the piston to move towards the proximal end only by the influence of gravity. In delivery systems such as pump systems, where the conduit is in contact with the fluid medicament for a substantial period of time, it is normal procedure to connect the plunger to the plunger rod in order to provide a controlled forward movement of the plunger thereby preventing the reservoir from emptying itself.
Some commercially available infusions pumps are adapted to include a user-fillable reservoir, which the user fills with the medicament prior to use. Such fillable reservoirs usually includes a piston having the back wall provided with an interior thread into which a pull rod is screwed. The piston can then be moved backwards inside the fillable reservoir, and the reservoir can be filled with medicine from a vial. Once the reservoir has been filled, the pull rod is disconnected from the piston and the reservoir is inserted into the medication pump.
Depending on the specific design of the pump drive system, a linear actuation member is adapted to engage the piston when a reservoir is inserted into the medication pump. Some coupling mechanisms rely on a positive locking of the linear actuation member to the piston where the coupling is maintained throughout the use of a single reservoir.
Other pump systems may rely on a simple abutment between the linear actuation member and the piston, i.e. the linear actuation member is not positively locked to the piston during pump use. However, in order to take the abovementioned self-emptying into consideration, the friction between the piston and the reservoir wall has to be considered to avoid the risk of free-flow. In commercially available user fillable reservoirs, such as the ones designed for use in insulin pumps marketed by Medtronic Minimed under the trademark PARADIGM, the piston and reservoir are designed such that sufficient friction is established between the piston and the reservoir wall.
Generally, it is desirable to use pre-filled reservoirs in pump systems. However, various conditions have to be taken into consideration in order to provide a reservoir which is suitable for long term storage of insulin. Conventional reservoir materials such as glass for the reservoir wall and specific rubber compositions for completely or partly constituting the piston are generally accepted for constituting insulin prefilled cartridges. Usually, prior to filling, conventional glass cartridges are subject to a siliconization process in order to reduce static friction between the cartridge wall and the piston.
Due to reduced friction compared to the above mentioned user-fillable reservoirs, conventional prefilled glass cartridges cannot be readily adopted in pump systems which has a linear actuation member which solely abuts the piston, i.e. does not provide a positive lock, and which at the same time is not provided with additional means for excluding free-flow.
Various different references describe mechanisms for ensuring disconnectable but positive locking of a linear actuation member and a piston situated in a reservoir.
However, due to the specific design of the infusion pump, total freedom with respect to the movements required for coupling and uncoupling a linearly moving actuation member and a reservoir comprising a piston is seldom obtainable. These limitations both arise due to the specific movements of the linear actuation member which may comprise axial only or axial as well as rotating movements, and due to the specific movements required when inserting or removing a reservoir into the reservoir receiving section of the infusion pump.
WO patent application No. 2005/002652 discloses an infusion or injection device being provided with a mechanism which secures the connection between a piston and an output member against axial displacement. The connection is established by axially displacing the output member and the piston relative to each other. Disconnection is obtained by relatively twisting the output member with respect to the piston.
According to the various embodiments shown in WO patent application No. 2005/002652, the connection between the actuation member and the piston is provided by a number of resilient engagement parts extending from the actuation member, where the end of the engagement parts cooperates with an internal thread formed in the piston. Due to the piston being formed of a flexible material, and due to the limited contact surface between the piston thread and the engagement parts of the actuation member, a connection is established which is rather flexible in the axial direction, leading to inaccuracies in the amount of fluid dispensed by the infusion pump.
The same deficiencies applies to the connection shown in U.S. Pat. No. 6,447,487 which specifically addresses the problem of free flow. Here a rigid actuation member frictionally engages an inner thread of a rubber piston. Due to the piston being elastically deformable, the engagement between the actuation member and the piston allows for a relative high degree of flexibility along an axis parallel to the dispensing movement.
U.S. Pat. No. 6,800,071 discloses a piston for a fluid medicament reservoir where the piston is formed by a flexible sealing part and a more rigid part inserted in the flexible part, thereby providing rigidity and stability to the flexible part. As discussed in this reference, some pump systems may have an occlusion detection system which uses the axial force on the drive train as an indicator of pressure within a reservoir. The provision of having a relatively stiff and incompressible piston improves the speed that an possible occlusion can be detected. However, the flexible piston part is provided with threads for threadedly coupling the piston to corresponding threads on a linear actuation member of the pump drive system. As the piston threads are formed in the flexible material of the piston, a connection is obtained that may induce some resilience between the piston and the linear actuation member, which again lead to inaccuracies in the amount of fluid expelled from the reservoir, and, the performance of the occlusion detection system is not optimal.
U.S. Pat. No. 6,569,126 and GB patent application No. 2,117,249 discloses syringes having a slideable piston inserted into the syringe. The disclosed syringes are provided with pistons having additional means for excluding rearwards movement of the piston. Neither of those references include disclosure of piston constructions having means for a well defined enhancement of the frictional engagement between piston and syringe wall during forward movement of the piston.
In view of the above, it is an object of the present invention to provide a drug infusion system having increased safety against free flow of fluid from the reservoir. A further object of the present invention is to provide a reservoir for containing a drug to be infused into a patient, wherein the the piston of the reservoir is provided with means for ensuring enhanced control of frictional forces between the piston and the reservoir wall.
Further objects and advantages of the present invention will be apparent from the below disclosure as well as from the description of exemplary embodiments.